Conventionally, when a rotor core (or a stator core) is manufactured by punching out annular core sheets from a magnetized thin metal strip and laminating the core sheets, the rotor core is uniformly fabricated by reducing a thickness deviation and a magnetic deviation in a circumferential direction. Specifically, each of the annular core sheets is relatively rotated by a predetermined angle (for example, the rotor core is rotated while the core sheets are laminated thereon) and the core sheets are laminated to a predetermined thickness (i.e., the rotor core is manufactured by rotating and laminating the core sheets).
Patent Literature 1 discloses a method of manufacturing a rotor core, in which angular positions of a punch and a die to make a shaft hole in a core sheet are changed by a predetermined angle (90 degrees in this example) to form a recessed key at a different position in each of the core sheets, and the core sheet just-provided with the recessed key is laminated on an assembly of the core sheets already laminated in the die while the assembly is being rotated by the predetermined angle, i.e., the core sheets are rotated and laminated.
Patent Literature 2 discloses a rotor core including projecting (convex) keys opposed to each other in a shaft hole, in which arc-shaped portions whose tangential directions are continuous with lateral sides of the keys are provided at both sides of the keys in a circumferential direction of the shaft hole so that stress concentrations at bottoms of the keys caused during a high-speed rotation of a rotor are reduced.